Supercapacitors are important because of their increasing role in powering many mobile, wearable and medical devices that help and improve peoples' lives. The main advantages of supercapacitors vs. batteries are fast charging and discharging, high power, and long cyclability. Supercapacitors can store energy with electrostatic reactions, such as in electric double-layer capacitors (EDLCs), which are made from carbon materials, or they can store energy based on faradaic reactions such as in redox (or pseudo) supercapacitors, made from transition metal oxides. The main figure of merit for supercapacitors is their capacitance in Farads (F) and their energy capacity in Joules (J).
Conventional supercapacitors 101 are made from three separate parts: anode electrode 102, separator wetted with liquid electrolyte 103, and cathode electrode 104, as illustrated in FIG. 1. The conventional electrode is fabricated from several components: metal foil 105 (such as copper, nickel, stainless steel, or aluminum) for current transfer, active material 106 (such as activated carbon (AC)) that stores the energy, and a polymer-based binder 107 (such as poly(vinylidenedifluoride)—PVDF, poly(tetrafluoroethylene)—PTFE, or Nafion) mixed with the active material 106 to adhere it to the metal foil 105. Disadvantages of using binder 107 are that it reduces the electrical conductivity of the supercapacitor, limits the thickness of the active layer and adds to the manufacturing cost. Activated carbon electrodes are typically thick, in the range of 100 μm and more. For example, one of the best super thin AC based supercapacitors, such as Seiko Model CPX3225A752D with 7.5 mF capacitance, has AC electrode thickness of about 200 μm, excluding the current collector. For this model, the entire stack of current collectors, activated carbon and, separator, but excluding the thickness of the environmental wrapping, is about 500 μm, which is too thick for many applications such as supercapacitors for smart cards and supercapacitors that can be incorporated with integrated circuits.
Redox (or pseudo) supercapacitors are made from transition metal, such as Ni, Mn, Co, and Ru oxides or hydroxides and are considered one of the best redox supercapacitor materials due to their high theoretical values for the specific capacitance. These redox supercapacitors are fabricated using transition metal oxide or hydroxide active material in form of powder, flakes, or nanoparticles and a polymer-based binder mixed with the active material to adhere them to a metal foil. The drawback of the binder is that it reduces the electrical conductivity of the supercapacitor and limits the thickness of the active layer. These supercapacitors have larger energy density than the activated carbon-based supercapacitors but have the same thickness disadvantages as the AC supercapacitors described above.
Among the transition metal oxides and hydroxides, NiO and Ni(OH)2 have been studied due to their natural abundance and low cost. A special version of redox supercapacitors in the form of nanoporous nickel has recently been disclosed by Rice University, which is described in patent application (WO 2013/119295 A1), which is incorporated here by reference, as well as a PCT filing PCT/US2015/024945 which is also incorporated here by reference. The above prior art disclosure describes a binder-free redox supercapacitor with a nanoporous nickel layer as the active material that is an integral part of the nickel metal foil. The nanoporous nickel has pore diameters on the order of 2 to 10 nm and is different from commercially available nickel foam which has pores with diameters on the order of 200 to 500 μm. The nanoporous nickel is electrochemically-etched from the nickel foil electrode and therefore does not require use of binders. As result, the nanoporous nickel based supercapacitors can be made very thin and still preserve the energy density advantages of the transition metal-based supercapacitors with binders.
For example, a symmetric redox supercapacitor 201 with a nanoporous nickel (NiO, NiF2 or Ni(OH)2) as the active material 202 and 207, as illustrated in FIG. 2A, can have electrodes 203 and 204 with thickness of 10 to 25 μm that also includes the current collectors 206 and 208. The current collectors 206 (and 208) are the remainder of the nickel foil that has not been etched (the unetched nickel section of a nickel foil). Therefore, the sum of the thicknesses of the stack comprising of first current collector 206, second current collector 208, first active material 202 (nanoporous nickel), second active material 207, and separator 205, but excluding the thickness of the environmental wrapping, can be in the range of 70 to 100 μm. The voltage range of this symmetric supercapacitor is up to 1.6 V and the volumetric capacitance (F/cc) and capacity (J/cc) is a few times higher than that of an activated carbon-based supercapacitor. The thickness, the voltage, and the energy of this supercapacitor are suitable for many applications such as supercapacitors for smart cards.
In another example of the prior art, an asymmetric redox supercapacitor 211 with a nanoporous nickel (NiO, NiF2 or Ni(OH)2) as the anode 212 active material and activated carbon as the cathode 217 active material, as illustrated in FIG. 2B, can have anode electrode 213 thickness of 10 to 25 μm that also includes the anode current collector 216 (the unetched nickel section of a nickel foil), and an activated carbon cathode electrode 214 thickness of about 100 to 200 μm, including the optional current collector 218. Therefore, the entire stack of first current collector 216, second current collector 218, active material (nanoporous nickel anode 212 and activated carbon cathode 217), and separator 215, but excluding the thickness of the environmental wrapping, can be in the range of 125 to 250 μm. Although thicker than the symmetric version of the nanoporous nickel supercapacitor, the asymmetric version has wider voltage potential, up to 2 V, and therefore better volumetric capacity (J/cc) than the symmetric version.
Therefore, there is a need for an asymmetric redox supercapacitor with nanoporous nickel as the anode active material and an alternative ultra-thin carbon-based cathode to reach the performance of the device described in FIG. 2B but with an electrode thickness of the device described in FIG. 2A.